Microcapsules consist of a core material surrounded by a coating or encapsulating substance which is normally a polymer. Microcapsules may consist of one or more spherical core particles surrounded by a coating, or the microencapsulated substance may exist as one or more irregularly shaped particles surrounded by a coating which may have spherical form, or the exterior of the microcapsules may be irregular in shape. In general, microcapsules are produced to provide protection for the core material and/or to control the rate of release of the core material to the surrounding environment. Also included within the term microcapsule are those in which the pharmaceutical agent is present as a solid solution in the coating and may be present at one or more points or portions of the microcapsule surface. The term microsphere has also been applied to the above-named microcapsules.
As suggested by Beck et al., U.S. Pat. No. 4,585,651, dated Apr. 29, 1986 which discloses pharmaceutical compositions comprising microparticles of a pharmaceutical agent incorporated in a biocompatible and biodegradable matrix material, the methods for preparation of microcapsules may be classified in three principal types:
(1) phase separation methods including aqueous and organic phase separation processes, melt dispersion and spray drying;
(2) interfacial reactions including interfacial polymerization, in situ polymerization and chemical vapor depositions; and
(3) physical methods, including fluidized bed spray coating, electrostatic coating and physical vapor deposition.
The distinguishing feature of phase separation microencapsulation is the initial production of a new dispersed phase containing the coating substance via some physical or chemical change. The dispersed coating phase ultimately surrounds and coats the core material which itself is also initially dispersed or dissolved in the continuous phase.
In one preferred type of phase separation, microencapsulation is carried out by addition of a non-solvent for the coating polymer and the core material to a solution of the coating polymer which contains dispersed or dissolved core material. This type of phase separation process comprises the following steps.
(i) A solution of coating material is prepared.
(ii) The core material is dispersed or dissolved in the coating solution. The core material may be solid or liquid and may or may not be soluble in the coating solution. The core material may also contain, in addition to any pharmaceutical agent, excipients such as antioxidants, preservatives, release-modifying agents, and the like. Any or all of the core material ingredients may be solid or liquid.
(iii) While stirring the composition of (ii), a non-solvent for the coating material and core material is added. The non-solvent must be miscible with or soluble in the coating solvent. Addition of the non-solvent causes the coating material to come out of solution in the form of a dispersed liquid phase comprising a concentrated solution of the coating polymer in the original coating solvent. In the case where the core material is soluble in the coating solution, the core material will also be present in the coating solution phase. In the case where the core material is not soluble in the coating solution, the newly created phase surrounds and coats the dispersed core phase. In this instance, a necessary property of the coating phase is that it wet the core phase in preference to the continuous phase.
(iv) The dispersion (iii) is added to the hardening solvent. The purpose of this solvent is to extract polymer solvent from the coating/core droplets formed in step (iii). After hardening, the microcapsules will exist as particles suspended in the hardening solvent. The microcapsules may then be recovered by filtration or other convenient means.
Kent et al., European Patent Publication Number EPO-052- 510, published May 26, 1982, discloses the microencapsulation of water soluble polypeptides in biocompatible, biodegradable polymers such as poly(lactide-co-glycolide)copolymers, also by a phase separation process utilizing an alkane solvent, and specifically exemplifies heptane as a hardening solvent.
The previously used hardening agents including hexane, heptane, cyclohexane and other alkane solvents leave substantial amounts of hardening agent residues in the microcapsules. Tests have shown that heptane hardened microcapsules typically contain 5-15% by weight of heptane. Since hardening agents can ultimately be released, low toxicity is of paramount importance for hardening agents used to produce microcapsules for pharmaceutical applications, and it would be advantageous to provide the same.
In addition, a further drawback in use of hydrocarbon hardening agents of the prior art is that they are flammable and therefore require the use of explosion-proof facilities for manufacturing microcapsules.
It has now been discovered that if volatile silicone fluids are used as hardening agents, the drawbacks of the prior art are overcome because of their very low toxicity and non-flammability characteristics. Microcapsules produced by the phase separation microencapsulation process are different and better than those of the prior art because the residual hardening agent content is very low, e.g., of the order of less than 2-3 wt %, preferably less than 1-2% and more preferably less than 1%. The results obtained herein are surprising because, while the coating material solvent is readily removable by vacuum drying, it has heretofore been the experience that residual prior art hardening agents, once incorporated into microcapsules, are not readily removed by drying because they are, by nature, not soluble in the coating material and therefore do not permeate through the coating material.
Volatile silicone fluids are unique because these fluids essentially are not incorporated into the microcapsules during the hardening step.
The improvement in phase separation microencapsulation thus provided by the present invention removes a major obstacle which in the past has prevented use of this technology to produce a drug delivery system.